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|Light balancing filters on digital|
Tungsten light is a dynamic-range killer, and in-camera white balancing can't fight it alone. There is a pervasive misconception that setting a white balance on the camera magically makes everything ok when shooting under tungsten or halogen light. Nothing can be further from the truth. No matter what the camera is set to, it is always dealing with the same incoming light: deficient in the blue channel. When the camera "white balances," from tungsten light, it has no option but to amplify the blue channel. This emphasizes noise: underexposed channels have noise already, and the blue channel is the weakest on most imagers. The result is that tungsten pictures - despite "white balancing" - end up unusually noisy. This problem gets worse when you increase the ISO because all channels (including the already-noisy blue one) get amplified more.
The solution that suggests itself is using an 80A filter. This is a blue filter originally designed for shooting daylight color film under tungsten light. The effect of using an 80A is that you suppress everything but blue light. On a digital camera set to daylight (or flash) color balance, the results of using an 80A filter are that:
1. Red, green and blue levels in the light source end up approximately equal, and white tungsten light looks white. Subjects look normal.
2. Blue channel noise is greatly reduced, even at high ISO settings.
3. Red-channel overexposure halos (blooming) around the edges of light sources diminish a bit.
4. Exposure time is increased by 2 stops (so 1/250 sec ends up being 1/60 sec, or 400 ISO becomes 100 ISO). It is best to use color-blind centerweighted metering.
In practical terms, there are three practical lessons:
1. If you have any interest in preserving the precious dynamic range of your camera, it is important to make gross adjustments to light temperature before the image hits the sensor. Remember, dynamic range is measured from the noise floor, which is affected by blue-channel noise.
2. Gross adjustments required to minimize blue channel noise reduce the effective sensitivity of the system. In fact, you will find that the system sensitivity is just as low as it is using daylight film with an 80A filter.
3. People who crow about how auto white-balancing works and how digital is "effectively" faster in low (tungsten) light ("just crank up the ISO") must have a very high tolerance for dark-area noise.
In addition to the 80A filter, other light balancing filters can come in handy for digital. The 81A is a very mild warming filter originally designed to cheer up cloudy days (hence the name, "Cloudy"). It is usable both for this purpose and as a general protective filter. The 82B is a very mild blue filter ("Morning & Evening") that helps counteract the very warm light cast as the sun rises and sets. Many people will find these allow quick, predictable corrections without pressing buttons, turning dials and otherwise trying to adjust presets. But in the grand scheme of things, the brownish filters are less important because they can be simulated after the fact using channels that are not as noisy as the blue channel.